Digital Phase Lock loops (DPLLs) are typically used to align chip internal clocks to a master input clock as shown in U.S. Pat. No. 4,795,985, granted Jan. 3, 1989 to Gailbreath, Jr. A digital phase lock loop may include a digital variable delay line and control logic to modify the digital variable delay line with up/down signals, which may be derived by comparing an input signal with a reference signal. Numerous examples of such DPLL functions exist, including U.S. Pat. No. 6,771,096, granted Aug. 3, 2004 to Meyers et al., which covers one form of control logic, and U.S. Pat. No. 5,982,713 granted Nov. 9, 1999 to Schenck et al., which describes a combination of buffers and switching capacitance to form a digital variable delay line.
DPLLs may be used to align the chip outputs to the master input clock as shown in U.S. Pat. No. 8,134,412, granted Mar. 13, 2012 to Karabatsos, or to synchronize the clocks from multiple units within a chip as described in U.S. Pat. No. 7,368,962 granted May 6, 2008 to Nakamuta et al. DPLLs may also be used to synchronize clocks between multiple chips. In U.S. Pat. No. 5,631,591 granted May 29, 1997, Bar-Niv describes synchronizing bus clock outputs from two chips, and in U.S. Pat. No 7,256,528 granted Aug. 14, 2007, Drost et al. describe synchronizing multiple chip's internal clocks; but in both cases, dedicated external clock or reference signals are needed to perform such synchronization. In order to ensure the sourced clocks are properly aligned, the reference signals may require careful board level layout to avoid creating uncorrectable differences in the synchronization circuitry. It would therefore be desirable to independently align the source to each of a plurality of chips in a system without the need for such external reference signals.